My Solution: Remopve the cloth, line up both the vertical and horizontal lines across the cone; a line that goes up through the center and aline that goes left to right through the ceneter so that both lines intersect at exactly the dead center of the cone. Mark these spots off, use a ruler and some tape and place a marker on the tolex where that line is. Put the Grill back on, Take some tape and run it down both the linesso that you get a + shape. Place your mic starting at the center and get a position taht you reallly like, using the + as a reference to the center. Once you get the mic posistiont that you really lilke, use more tape to set up a guide as to where to pu thte mic, this can be a small square, or even a large rectangle along the horizontal plaine so that you can move where the mic is from the cone without worrying about finding the dusctcap. Afterwards you can remove any of the excess tape that you don't need, just mark it up so that you know wherer the poisitions that you like are. Its really cool if you use painter's tape becuase it looks really professional and comes off extreemly easily without ruining wherer you stick it to (think of them as post it notes for your cab)
I find that the cloth contributes to the sound, unless you have a metal grill, but even they can alter the sound depending on how big the openings are, actaul cloth dampens the highs in a way that I find EQ can't, so I like to try to do as much as I can with the grill inclduing marking the center before completely removing it.